The present invention relates to a device for measuring electrostatic properties of materials so that the materials can be quantitatively ranked. Determining the electrostatic discharge properties and characteristics of certain materials can be critical in applications involving semiconductors, which are sensitive to static discharge and can be easily ruined by such discharges. Electrostatic discharge properties can also be critical where fluids are used which might be ignited by static discharges. In the aerospace industry, the determination of electrostatic discharge properties of materials can be of critical importance.
Prior art testing methods have been unsatisfactory for several reasons. Typically, they utilize corona charging techniques and provide no concurrent information concerning the triboelectric charging propensities and sensitivities of the materials being tested. Yet to be fully determinative, it is necessary to have information and testing capabilities for both.
Another disadvantage of prior art methods is that they have provided no direct comparison among properties of different materials. The current practice of quoting electrostatic properties in qualitive and unrelated terminology makes application of materials difficult. It is important to be able to rank materials directly with competing materials in use conditions.
In trying to compare different materials using prior art equipment and methods, it has been necessary and critical, but very difficult, to calibrate the voltage sensor, and to maintain accurate environmental controls, since testing has been done on each sample separately. Other prior art disadvantages obtain when sample surface integrity is not preserved.
The traditional methods used for electrostatic discharge tests on materials may roughly be classified as: (1) resistivity tests, (2) shielding tests, (3) discharge tests, and (4) triboelectric tests. Tests which measure volume and surface resistivities include such methods as ASTM D-257 and ASTM D-991. Resistivity tests are both simple and reproducible, but the results of resistivity tests seem to have no simple correlation with electrostatic discharge behavior. Shielding tests, such as ASTM F 365-73T, only measure transient effects, so are too specific to be useful for general characterization of electrostatic behavior. Discharge methods, including ASTM D-4238 and FED STD 101C Method 4046.1, are widely used because they provide direct and sensitive characterizations of electrostatic discharge properties. They characterize the dissipative electrostatic properties of materials, but ignore an important charge generation property of materials, namely, tribocharging. Tribocharging methods include the AATCC Test Method 134-1979 and the KSC Triboelectric Test Method. Both these methods address the generation of charge triboelectrically, but the latter also measures charge decay.
With regard to the patent literature, U.S. Pat. No. 2,421,430 (Ott, issued June 3, 1947) discloses a device for measuring static electricity present in textile materials utilizing the triboelectric method. A rotating plate is featured, but is utilized to impart frictional contact with the material in order to affect the static electrical charge on the material.
U.S. Pat. No. 3,225,299 (Middendorf, issued Dec. 21, 1965) discloses a tachometer for measuring rotational speed by measuring the rate of transfer of electrostatic charge desposited on the dielectric surface of a rotor, from one electrode to another.
U.S. Pat. Nos. 3,727,125 and 3,943,437 (Mourier, issued Apr. 10, 1973, and Mar. 9, 1976) disclose the use of the corona effect to determine discharge properties of materials placed on a rotating plate and alternatively charged and tested for electrical content.
U.S. Pat. No. 3,544,889 (Alauzet et al., issued Dec. 1, 1970) discloses the use of the corona effect to test samples on a rotating plate.
U.S. Pat. No. 3,406,344 (Hopper, issued Oct. 15, 1968) is believed to disclose nothing further than the above.
Non-patent literature of possible interest includes:
Baumgartner, G. and Haverman, R., "Testing of Electrostatic Materials FED. STD. 101C, Method 4046.1", in Electrical Overstress/Electrostatic Discharge Symposium Proceedings, pp. 97-103. Philadelphia: The EOS/ESD Association, October, 1984. PA0 Berbeco, George R., "Characterization of ESD Safe Requirements for Floor Surfaces.", in Electrical Overstress/Electrostatic Discharge Symposium Proceedings, pp. 124-130. Orlando, Fla.: IIT PA0 Gompf, Raymond H., "Triboelectric Testing for Electrostatic Charge on Materials at Kennedy Space Center.", in Electrical Overstress/Electrostatic Discharge Symposium Proceedings, pp. 58-63. Philadelphia: The EOS/ESD Association, October, 1984. PA0 Tse, Ming-Kai and Suh, Nam P., "An Electrostatic Charge Decay Technique for Nondestructive Evaluation of Nonmetallic Materials.", in International Advances in Nondestructive Testing, Vol. 9, pp. 193-226. Gordon and Breach, Science Publishers, Inc., n.p., 1983.
Research Institute, Sepember, 1982.
A need therefore remains for a method and apparatus for measuring electrostatic properties of materials such that the materials can be easily, accurately, and repeatably ranked quantitatively with respect to both electrostatic charging and triboelectric charging. Further, such a method and apparatus must not be overly sensitive to instrument calibration or environmental control conditions, and must preserve sample surface integrity for determination of tribocharge generation propensity.